The present invention relates to integrated optical structures using guided propagation of optical radiation and more particularly to directional couplers. By bringing together over an appropriate length two optical monomode waveguides, an exchange of energy may be observed. The evanescent waves create coupling between the waveguides and it is sufficient for the guided waves to have propagation speeds sufficiently close to one another for a transfer of energy to take place. With lossless waveguides, a total transfer may be achieved which consists in collecting at the output of the second waveguide all the photons injected at the input of the first guide. By taking as basis the electro-optical effect of the material used for constructing the integrated optical structure of a directional coupler, the energy transfer may be controlled electrically. This electric control may assume two aspects according as to whether the electric field is electrostatic in origin or produced by an electromagnetic wave capable of accompanying the photons in their guided progression. When the electric field is stationary, the result of the transfer does not depend on the distribution of the photons so that the energy injected is switched or commutated depending on the electric control voltages applied to the electrodes of the structure. On the other hand, when the control depends on an electromagnetic wave which accompanies the photons in their guided progression, each photon undergoes a fate different from its neighbors, for the directional coupling which concerns it remains velocity matched to an element of the invariable composition conveyed by the electromagnetic wave.
By providing the electric control for a directional coupler formed on an electro-optical substrate by means of a colinear electromagnetic line through which travels a sinusoidal wave, from a continuous light flux there may be obtained an intermittent flux whose pulsations have a duration very much less than those obtained with blocked-mode injection lasers. The provision of velocity matched directional coupling allows light pulse sources to be obtained of very short duration from continuous emission sources. This technique opens the way to time multiplexing-demultiplexing devices which may process signals of a duration less than a picosecond.
The operation of an integrated optical structure using velocity matched directional coupling depends on precise matching of the propagation velocity of the electromagnetic line and that of the guided photons. By appropriately dimensioning the electrodes of the electromagnetic line and by choosing media with suitable dielectric constants a still imperfect structure is obtained, for the manufacturing process introduces errors which it is then difficult to compensate for in order to obtain perfect coincidence of the velocities over a coupling distance of several centimeters.